There are considerable variations among species in the response of serum cholesterol to the feeding of dietary cholesterol; in the human serum cholesterol concentration rises only slightly when cholesterol is fed. Three mechanisms are involved in this protective device - the negative feedback of exogeneous cholesterol on cholesterol synthesis, increased bile acid production, and a limitation in the ability to absorb cholesterol. Three aspects of the influence of dietary cholesterol on cholesterol metabolism are to be investigated: 1. A measurement of the ability of the baboon to absorb dietary cholesterol. 2. An assessment of the origin of serum cholesterol in primates. 3. An analysis of the effects of thyroid hormones on the response of the intact animal to cholesterol feeding.